Acanthamoeba species were first found in an ocular infection in 1973. Since that initial non-contact lens associated case of infection (Acanthamoeba Keratitis or AK), most cases have been associated with contact lens wear. The first documented series of cases were associated with the use of water with salt tablets, with enzyme tablets or wearing lenses while swimming or in a hot tub. Following those incidents, the annual rate of infection in the United States was reported by Schaumberg and co-workers in 1988 as approximately 2 cases per million. In the UK, Seal reported in 2003 levels at one per thirty thousand (approximately 33/million) as the base line value. The incidence rate most often quoted for the UK incidence is 20 per million.
During 2005-2007, workers at the University of Illinois Chicago (UIC) reported an outbreak that was referred to the Centers for Disease Control (CDC). CDC initiated a multistate investigation. An expanded investigation by CDC indicated that the risk of Acanthamoeba keratitis (AK) was increased in the use of a single product, Abbott Medical Optic's (AMO) Complete Multi-Plus. The reported correlation with this specific product was attributed to components of the formulation causing the organism to go from the trophozoite form to the cyst. The cyst is more resistant to most disinfection options.
Since the withdrawal of the AMO product, the incidence in the US has not returned to the levels reported in a 1998 study by D. Schaumberg based on the results of a review of the incidence from key ophthalmology centers. The current rate is estimated to be in the order of 12-20 cases per million in the US. CDC has continued to follow up on the incidence of AK.
It has been hypothesized that this is a result of changes in the treatment of potable water. The levels of chlorine used for disinfection were decreased to reduce the incidence of chlorinated by-products.
Understanding the exposure rates for contact lens wearers from the environment is important in developing approaches to testing products for disinfection efficacy. Researchers especially in the UK have regularly reported significant evidence of contamination of the water supplies in homes usually using techniques which swab the inside of the pipes and faucets in homes especially in “stored” water. However, environmental studies in the US have showed low levels of organisms in the water column in lakes and rivers, often measured in organisms per 100-200 ml. Levels were often below ten. A study carried out in the water supplies in Fort Worth, Tex. and presented at the University of Houston showed no organisms in “grab” samples during storm-free periods from lakes used as public water supply. Low levels of Acanthamoeba were detected after rain periods with significant run off, and in small ponds without significant water flow. In the same study, no Acanthamoeba were recovered from flowing water from homes.
The primary standard to test microbiocidal disinfection efficacy of contact lens care products is ISO 14729.13 This standard calls out the testing against 5 organisms, two gram negative bacteria, Pseudomonas aeruginosa, and Serratia marcescens, a gram positive bacteria, Staph. aureus, a yeast, Candida albicans, and a mold Fusarium solani. During the development of this standard in the 1990's (developed after the outbreak of the 1980's), a discussion was included for the reason the Acanthamoeba was not included as a part of this test protocol. In 2014 a second standard ISO 18259:2014 has been put in place under ISO testing care products in the presence of lenses and cases. This new standard includes the same five organisms as ISO 14729. Acanthamoeba is not included as a test organism in the new standard.
The ISO disinfection standard 14729 has been the primary standard accepted by most jurisdictions around the world. The United States under FDA has currently no separate standard, and this standard is included in the ANSI Z-80-18 standard on contact lens care product.
After the 2007 incident first observed at UIC, there have been a number of approaches to evaluate efficacy against various species of this genus. During discussions carried out at a Workshop sponsored by the FDA in January 2009, a single item received a consensus recommendation to be included into a testing standard. This was a measure of conversion of trophozoites to cysts (or encystment). This has been included in an ISO standard (ISO19045) which published in 2015. This new standard does not include any acceptance criteria although the difference between the positive control (greater than 30% encystment) and negative controls (less than 5% controls) were observed in the inter-laboratory of this standard.
The development of measurements of disinfection efficacy has had much more difficulty. A number of Acanthamoeba species have been examined using a variety of organism growth conditions, test conditions and recovery techniques. These difference have led a variety of results for individual care products. Only recently has a single approach to determination of trophozoites has been selected by experts on ISO TC172/SC7/WG9 for validation using 2 species. The experts were in the process of developing a protocol to run an inter-laboratory ring test to validate the methodology at the end of 2015.
The second phase of the development of methods is to measure disinfection efficacy of the cyst form of Acanthamoeba. Procedures and protocols have not been finalized inside the expert community. In general numerous workers have published results for disinfection efficacy against Acanthamoeba species. Given the non-standard testing conditions; some differences have been reported.
Currently, even with two microbiology workshops and recommendations from ophthalmic panel meetings, there are no testing requirements for Acanthamoeba required during the approval of a disinfection in the United States. At this time even the procedure for the evaluation of products causing encystment (ISO 18259) has not been adopted in the US. However, there is a perceived need to have care systems that are “effective” against Acanthamoeba. 
While encystment was related to the recall of a product in 2007, there are no criteria in the ISO standard 18259. Any new product should not cause any significant encystment under test protocol.
Beyond the difficulty in finding appropriate testing methodology, setting appropriate criteria for effectiveness have not been addressed. There are three different types of organisms that are a concern in lens care—bacteria, fungi and amoeba. They occur in significantly different rates. The overall observed rate for all organisms in daily wear is approximately 4 in 10,000, and 20 per 10,000 in extended wear. Most frequently bacterial infections are observed followed by fungi at much lower levels. As a comparison the incidence of Acanthamoeba infections are nearly 200 to 1000 times less frequent than the overall infection rate. In terms of the ability of most disinfection, bacteria are more easily eliminated compared to fungi and both much more easily eliminated compared to Acanthamoeba; especially the cyst form.
The current primary acceptance criteria in the global standard, ISO 14729, for bacteria is a reduction of an inoculum of approximately one million (106)/ml by three log to one thousand/ml (103). Fungi are expected to be reduced one log from approximately 1 million (106) to one hundred thousand (105); or one log. If products do not meet the primary criteria, they can be acceptable for use in a disinfection regimen by qualifying by demonstrating disinfecting activity for bacteria and stasis for fungi followed by a simulated use of the product. In the development of criteria, given the requirements for efficacy against fungal organisms and the higher risk, justifying requirements for Acanthamoeba equal to or of above the requirements for fungi in current standards may be difficult.
The current approaches in the draft of ISO19045-218 to testing the disinfection efficacy are undergoing interlaboratory evaluations for trophozoites. The inoculum for these tests is lower (104-105) than that used for bacteria and fungi. While no criteria has been identified, a reduction of at least one log would be desirable for reduction of trophozoites under test with the solution, and no growth or stasis for inclusion in a simulated use test where the recovery is less the 10 organisms/ml in the general procedure outlined in 14729. Results from testing under the conditions suggested in ISO 18259 should show reduction and no increase with lenses and cases over time.
At this time there are no accepted protocols for disinfection of Acanthamoeba cysts. It is expected that the major considerations in developing methodology will be the process for forming the cyst form. It is likely that the inoculum levels will be determined by results obtained in the evaluations used for the ISO 19045-1 and 19045-2. Since the cyst is much more resistant to disinfection the disinfection efficacy is likely to be lower than that obtained with trophozoites. This presents a difficulty. Cyst forms of Acanthamoeba can revert to the trophozoite form and both are potentially infective. Ideally products would have similar disinfection efficacy for both trophozoite and cyst forms.
In the United States, there continues to be no requirement for testing Acanthamoeba even though recommended at the microbiology workshops and by the Ophthalmic Panel. Researchers have evaluated products, and many manufacturers have studied their own products and competitive products for activity under a number of different testing protocols.
In Europe, manufacturers having suitable data, have included claims for effectiveness for efficacy against Acanthamoeba on the product label. Other jurisdictions vary in their acceptance of specific claims.
Hydrogen peroxide (H2O2) is a widely accepted disinfectant that is used in a number of settings including disinfection of contact lenses. Although some activity has been demonstrated against acanthamoeba, only limited log reductions after long contact times have been reported with 3% H2O2 systems which employ concurrent peroxide neutralization schemes. Due to the risk of acanthamoeba keratitis, a system that can demonstrate much greater activity in this area would be advantageous having the potential to create a claim supported competitive advantage.